The purpose of this ongoing study is to create oxygen sensitive probes for in vivo use, and to use EPR to study the mechanism of oxygen interaction with the char. So far, the chars are proving to be a rich source of oxygen-sensitive probes, and may provide viable sensors for monitoring oxygen concentrations in cells or intracellular space in living tissues. Various carbonaceous materials (carbohydrates, polymers, cellulose) are being tested for oxygen sensitivity.